The present invention relates generally to the field of welding and, in particular, to a new and useful method of welding boiler tube wall panels using lasers.
Boiler tube wall panels or membrane panels of all types can be welded by the present invention. Such panels are either welded together, sided-by-side tubes with tangent tube welds therebetween, or tubes alternating with narrow flat plates called membrane bars, or round rods or bars to form a gas-tight wall. The tubes are generally about 1½ to 3 inches OD with wall thicknesses of about 0.15 to 0.30 inches. The thickness of the plates between the tubes is about ¼ to ⅜ and they can be up to about 3 inches wide. The plates are thus zero to 3 inches wide, the zero dimension indicating no plate present, i.e. a tangent tube weld, between adjacent tubes. The rod or round bar is about ⅛ to ½ inches in diameter. Welds needed to assemble such boiler tube wall panel parts must be from about 10 to 60 feet long or more since the panels can be this length. The panels can also be as narrow as only two tangent welded tubes and up to 8 feet wide and of the tangent welded, membrane plate or round bar types.
Tube wall panels can be made of various metals that range from inexpensive carbon steel to chromium molybdenum alloys, to composites with a metallurgically bonded stainless or Inconel (a trademark) sheath over carbon steel. Examples of the alloys that are used are Croloy 2¼ (2¼ Cr-1 Mo), also known as SA-213 T22, and another alloy designated SA-213 T23.
For a general description of the characteristics of boiler tube wall panels and welding processes currently used in making them, the reader is referred to Chapters 22 and 38 of Steam/Its Generation and Use, 40th Edition, The Babcock [[and]] & Wilcox Company, Barberton, Ohio, U.S.A., ©1992, the text of which is hereby incorporated by reference as though fully set forth herein.
It is known to use SAW (submerged-arc welding) and GMAW (gas metal-arc welding) to weld boiler tube wall panels together. These processes are slow and lead to distortion of the panels since they impart much heat to the tubes and/or plates or rods being welded together. Control of these processes is difficult due to the need to position both arc and weld wire through flux (SAW) while shielding the welder from arc burns (GMAW). Welds must also be made from both sides of the panel to produce fillets on both sides of the panel. This in turn requires significant time and effort to flip over the long, unwieldy panels in order to weld the back sides of the panels.
It is also generally known that lasers, in particular CO2 and Nd:YAG lasers, can be used to weld various types of metals to each other. See, for example, four articles all entitled “Laser Welding” at http://www.laserage.com/welding.htm, http://www.lmclaser.com.au/welding.htm, http://www.convergentprima.com/Laser_Applications/Laser_Beam_Machining/Welding/welding.htm, and http://www.alspi.com/welding.htm; “Laser Welding Article” at http://www.uslasercorp.com/Envoy/welding.htm; “Nd:YAG laser welding (March 2001)” at http://www.twi.co.uk/j32k/protected/band—3/kspah003.html;”“Carbon dioxide laser” (February 2000) at http://www.twi.co.uk/j32k/protected/band—3/kspah002.html; and “Application Experiences with Laser Beam Welding” at http://www.alspi.com/Isrweld.htm.
Laser welding has been used to make welds in thin sheet metal or to make spot welds, but not to weld together parts of a boiler tube wall panel which are quite thick in comparison and require deep, gas-tight welds.
Also see U.S. Pat. Nos. 4,990,741; 6,300,591; 5,760,365; 5,229,571; 6,211,483 and 5,563,391.
A need remains for an effective and improved way to weld together the parts of a boiler tube wall panel.